This invention relates to a stent to be placed in a lumen of a human body or an animal.
As well known, a shape memory alloy, such as a Ti—Ni alloy, exhibits a remarkable shape memory effect in association with martensitic reverse transformation. It is also well known that the shape memory alloy exhibits excellent superelasticity or pseudoelasticity in association with stress-induced martensitic transformation caused by strong deformation in a parent phase, that is, austenite phase after the reverse transformation. The superelasticity is observed in a number of shape memory alloys. Among others, the superelasticity is remarkable in the Ti—Ni alloy and a Ti—Ni—X alloy (X═V, Cr, Co, Nb, or the like) obtained by substituting an element X for a part of the Ti—Ni alloy.
The shape memory effect of the Ti—Ni alloy is described in U.S. Pat. No. 3,174,851 (hereinafter referred to as a patent document 1). The superelasticity of the Ti—Ni alloy is described in Japanese Unexamined Patent Application Publication (JP-A) No. S58-161753 (hereinafter referred to as a patent document 2).
The shape memory effect and the superelasticity of the Ti—Ni—X alloy are described in Japanese Unexamined Patent Application Publications (JP-A) Nos. S63-171844 (hereinafter referred to as a patent document 3) and S63-14834 (hereinafter referred to as a patent document 4) for a Ti—Ni—Nb alloy and in U.S. Pat. No. 4,770,725 (hereinafter referred to as a patent document 5) for a Ti—Ni—Nb alloy. As compared with the Ti—Ni alloy, the Ti—Ni—Nb alloy has a characteristic that transformation temperature hysteresis is increased by imposing a stress. Therefore, the Ti—Ni—Nb alloy is put into practical use as a joint for reactor piping.
Angioplasty using a stent is a technique for treating occlusion or narrowing of a blood vessel or a heart valve. The stent is a mesh-like metal tube or tube to be placed in a living body in order to prevent re-narrowing of a narrow portion, such as a blood vessel, after it is radially expanded. The stent is folded into a small size and mounted to an end portion of a catheter. After introduced into the narrow portion together with the catheter, the stent is released from the catheter and radially expanded to be attached to an inner wall of a lumen such as a blood vessel.
For example, in case of PTCA (percutaneous transluminal coronary angioplasty), the stent is radially expanded following a blood vessel expanding operation by inflation of an internal balloon set on an inner wall of the catheter. The stent is called a balloon expandable stent and formed by the use of a metal such as stainless steel or tantalum.
On the other hand, in order to prevent rupture of an aneurysm which may result in a subarachnoid hemorrhage or the like, blood supply to the aneurysm is stopped. As one of techniques for stopping the blood supply, use is made of embolization in which a metal coil such as a platinum coil is implanted into the aneurysm to form a blood clot (thrombus). However, it is pointed out that a part of the blood clot may possibly be released from the metal coil and carried by a bloodstream to a periphery to block a blood vessel. In order to avoid such undesired phenomenon, consideration is made about a covered stent technique in which the aneurysm is embolized by the use of a graft. In this case, simultaneously when the stent is released from the catheter, the stent is radially expanded by its own spring function to press the graft against a blood vessel wall. Such stent is called a self expandable stent. For the self expandable stent, a material having an excellent spring characteristic is desired.
The superelasticity in the Ti—Ni alloy is a behavior that, at a temperature above a reverse transformation finish temperature (Af point, austenitic transformation finish temperature) thereof, the alloy which has been deformed under an external load is recovered into an original shape simultaneously when the external load is released. A recoverable deformation is as high as about 7% in case of an elongation strain. For use as the stent, the alloy is formed into a hoop shape slightly greater in diameter than the lumen where the stent is to be placed. The stent is radially contracted and mounted to the catheter. Simultaneously when the stent is released from the catheter, the stent is autonomously recovered into the diameter of the original hoop shape to be brought into tight contact with the lumen such as the blood vessel. Thus, the alloy has the Af point lower than a living body temperature (37° C.) and always exhibits the superelasticity at the living body temperature.
As well as the above-mentioned characteristics, the superelastic stent has several problems. For example, its own spring function may damage the blood vessel wall and its autonomous shape recovery may cause a positioning error in the lumen. Therefore, it is difficult to use the superelastic stent in a blood vessel system such as a coronary system.
The stent for use in PTCA is preferably made of a metal material having a low spring function and a high rigidity. However, use of such material is disadvantageous in that a force urging a lumen wall outward is weak to cause a positioning error following blood vessel pulsation.
In view of the above, proposal has been made of a stent using a shape memory alloy.
Japanese Unexamined Patent Application Publication (JP-A) No. H11-42283 (hereinafter referred to as a patent document 6) discloses that a Ti—Ni—Nb alloy is applied to a stent. Specifically, the above-mentioned publication describes that the stent made of a Ti—Ni—Nb alloy and having a low Young's modulus upon shape recovery and a high Young's modulus upon shape deformation under an external load is obtained if the ratio of the stress on loading to the stress on unloading at the respective inflection points on the stress-strain curve is at least about 2.5:1. This stent exhibits superelasticity at the living body temperature after it is released from the catheter but does not sufficiently achieve desired positioning of the stent as required in PTCA.
The present inventors have already proposed a stent obtained by slotting in Japanese Unexamined Patent Application Publication (JP-A) No. H11-99207 (hereinafter referred to as a patent document 7). In detail, the patent document 7 proposes the stent which exhibits no shape memory effect at the living body temperature during insertion into the living body and exhibits superelasticity after shape recovery by inflation of a balloon. In the embodiment in the patent document 7, the stent made of a Ti—Ni alloy or a Ti—Ni—X alloy (X═Cr, V, Cu, Fe, Co, or the like) is subjected to strong deformation to thereby elevate a recovery temperature. However, in case of the stent obtained by slotting as shown in the patent document 7, the strong deformation is performed merely by accommodating the heat-treated stent into the catheter. Therefore, depending upon a slot shape, sufficient deformation is difficult and sufficient effect is not obtained.